The present invention relates to cathode ray tubes, and to magnetic deflection yokes used in such devices; in particular the invention relates to magnetic compensation for CRT deflection yokes and the like.
Cathode ray tubes (CRT's) are used extensively in the communication of visual images, such as in television receivers, data processors, word processors, and many other such devices. In some types of CRT's, the movement or scanning functions of the electron beam is controlled by an external deflection yoke which surrounds the neck portion of the vacuum tube. Due to the angle at which the electron beam impinges upon the curved, phosphorescent tube surface, distortion of the resultant images can occur, particularly at the marginal edge of the screen. Barrel and pin-cushion distortion are worsened in compact (short-necked), wide-angle tubes, such as those having a sidewall inclination in the range of 110.degree.. Further, the effect of this distortion is particularly critical in the display of alphanumeric symbols, as severe distortion can cause difficulty in reading, or even misinterpretation of the displayed images.
Because of the above-noted distortion effects, some CRT displays now utilize stator yokes, with built-in circuitry to reduce such image distortion. However, such constructions are many times more expensive than the typical saddle-torroid type of deflection yokes, and for this reason such yokes are very frequently used, despite the prevalent distortion. It is to this problem that the present compensator is addressed, and for which it is adapted to be used.
One type of compensator previously used with CRT's having external deflector yokes is shown in FIG. 1 of the drawings, and comprises an encirclement for the neck of the CRT which carries a plurality of outwardly-extending, plastic-coated pins. Each such pin has a pair of spaced-apart magnets frictionally retained on the outer end thereof by an interference fit over the plastic-coated pins. The inner ends of the pins are fixedly anchored in the forward collar of the deflection yoke, and the magnets can be rotated axially on the pins. To alleviate distortion in screen-displayed images, the CRT is activated with a test pattern, and an operator manually rotates the magnets until distortion has been minimized. In this type of compensator, the magnets are very difficult, if not impossible to adjust accurately, since they are in effect jammed into their respective pins and do not rotate smoothly increment-by-increment upon their pins. In addition to being rather inaccurate, this type of adjustment is a tedious, time-consuming procedure. Furthermore, and most disadvantageously, vibrations and other jolts or jars experienced by the CRT during transport, installation, and the like, tend to jar or jostle the magnets out of adjustment. Even the inevitable exposure to heat fluctuations during normal operation can result in changes of position in the magnets, altering their preset, adjusted compensation effect.